An edge-coupled microstrip transmission line filter can be used to implement a bandpass filter. However, an edge-coupled microstrip transmission line filter causes spurious, repeating passbands at the harmonic frequencies of the filter, which cause noise that can adversely affect devices near the filter. A low pass filter can be cascaded from the edge-coupled microstrip transmission line filter to reduce this noise, but the additional filter only reduces noise associated with the first harmonic frequency of the edge-coupled microstrip transmission line filter, while increasing the size and loss of the bandpass filter and contributing to noise associated with other frequencies. A uniplanar compact photonic-bandgap structure can be used as a ground plane in the edge-coupled microstrip transmission line filter to introduce a periodic disturbance that rejects the spurious passbands of the edge-coupled microstrip transmission line filter, but this structure is difficult to realize, since it must be isolated from other ground conductors to be effective. Modulating, in a sinusoidal pattern, the strip widths of a parallel-coupled transmission-line filter having a constant ground plane can reduce passband harmonics, but at the cost of insertion loss in the passband.